1. Field of the Invention
The invention pertains to the field of semiconductor optoelectronic devices. More particularly, the invention pertains to high-power high-brightness semiconductor diode lasers with a narrow beam divergence and to wavelength-stabilized semiconductor diode lasers.
2. Description of Related Art
There is a need in high-power power semiconductor diode lasers for numerous applications including, but not limited to material processing, projection television, frequency conversion, etc. For these applications high power and high brilliance (power emitted in a unit solid angle) are of key importance.
Conventional prior art edge emitting laser. In this approach a narrow waveguide supporting only a single localized waveguide mode is fabricated. Then the gain medium placed in this waveguide interacts predominantly with this mode. Once the feedback through the reflecting facets or distributed feedback mechanism is provided the optical power accumulates in the waveguide, the stimulated emission lifetime shortens and, once the modal gain overcomes total losses, the lasing starts. This approach has severe limitations. First, the output power is limited by the catastrophic optical mirror damage, and all technological improvements including facet passivation, zinc diffusion, or proton bombardment still have limitations in optical power density. To achieve higher power by keeping the same power density one needs using broad waveguide and wide stripe lasers. However, the lasing from broad area lasers is typically multimode and also suffers from beam filamentation which renders the laser radiation not focusable. Attempts to stabilize fundamental mode lasing are further complicated by the fact that the high-order modes, propagating at a larger effective tilt angle to the axis of the waveguide, (e.g. high-order vertical modes), experience a higher facet reflectivity, the power accumulated in these modes is higher, the stimulated emission lifetime is shorter and the lasing can be initiated through these modes. Even if the lasing starts via the fundamental mode, or example, due to the higher optical confinement factor for these mode, once the excitation density is increasing, high-order modes evolve due to the special gain depletion and the gain spectrum hole burning effects for the fundamental mode. Once the objective is to obtain a high optical power focused onto a small spot, then a two-stage approach, where semiconductor diode laser operates as a pump source for pumping solid state laser and then an optical beam emitted from the solid state laser is used, however this is extremely expensive. Therefore there is a need in the art in semiconductor diode laser allowing high power narrow beam divergence single vertical mode lasing.
Furthermore once an objective is just to pump a solid state laser by a semiconductor diode laser, a requirement on wavelength stabilization arises, as the photon energy corresponding to the lasing wavelength must be in a relatively narrow resonance with a relevant optical transition in the active medium of the solid state laser.
A requirement on wavelength stabilization of a semiconductor diode laser may arise also for frequency conversion application, if a non-linear crystal has a relatively narrow energy band, in which the conversion efficiency is high.
Thus a need in broad waveguide lasers operating in a single mode, allowing also wavelength stabilization exists.
A novel concept allowing such a device has been proposed in the U.S. Pat. No. 7,421,001, filed Jun. 16, 2006, entitled “EXTERNAL CAVITY OPTOELECTRONIC DEVICE”, issued Sep. 2, 2008, and in the U.S. Pat. No. 7,583,712, filed Jan. 3, 2007, entitled “OPTOELECTRONIC DEVICE AND METHOD OF MAKING SAME”, issued Sep. 1, 2009, both by the inventors of the present invention, whereas these both patents are incorporated herein by reference.
FIG. 1(a) shows a schematic diagram of a semiconductor diode laser (100) disclosed in the U.S. Pat. No. 7,421,001, by the inventors of the present invention. The device comprises a substrate (101), a waveguide (103), and a top cladding layer (129). The light generated within the waveguide propagates along the waveguide, which is shown schematically by the dashed line (104). Light propagating along the waveguide leaks to the substrate (101), propagates through the substrate, is reflected back from the back surface (131) of the substrate, and returns to the waveguide (103). Light in the substrate forms a tilted optical mode, or tilted wave (134). As the substrate thickness, which typically ranges from 50 to 300 micrometers significantly exceeds the thickness of the waveguide (103), the output light comes mainly from the substrate. The output light is emitted in two vertical lobes (145).
FIG. 1(b) shows a schematic diagram of a device (100) in more detail. The substrate (101) is formed from any III-V semiconductor material or III-V semiconductor alloy. For example, GaAs, InP, GaSb. GaAs or InP are generally used depending on the desired emitted wavelength of laser radiation. Alternatively, sapphire, SiC or [111]-Si is used as a substrate for GaN-based lasers, i.e. laser structures, the layers of which are formed of GaN, AlN, InN, or alloys of these materials. The substrate (101) is preferably doped by an n-type, or donor impurity. Possible donor impurities include, but are not limited to S, Se, Te, and amphoteric impurities like Si, Ge, Sn, where the latter are introduced under such technological conditions that they are incorporated predominantly into the cation sublattice to serve as donor impurities.
The n-doped bottom cladding layer (122) is formed from a material lattice-matched or nearly lattice-matched to the substrate (101), is transparent to the generated light, and is doped by a donor impurity. In the case of a GaAs substrate (101), the n-doped cladding layer is preferably formed of a GaAlAs alloy.
The n-doped layer (124) of the waveguide (120) is formed from a material lattice-matched or nearly lattice-matched to the substrate (101), is transparent to the generated light, and is doped by a donor impurity. In the case of a GaAs substrate, the n-doped layer (124) of the waveguide is preferably formed of GaAlAs alloy having an Al content lower than that in the n-doped cladding layer (122).
The p-doped layer (126) of the waveguide (120) is formed from a material lattice-matched or nearly lattice-matched to the substrate (101), is transparent to the generated light, and is doped by an acceptor impurity. Preferably, the p-doped layer (126) of the waveguide is formed from the same material as the n-doped layer (124) but doped by an acceptor impurity. Possible acceptor impurities include, but are not limited to, Be, Mg, Zn, Cd, Pb, Mn and amphoteric impurities like Si, Ge, Sn, where the latter are introduced under such technological conditions that they are incorporated predominantly into the anion sublattice and serve as acceptor impurities.
The p-doped cladding layer (129) is formed from a material lattice-matched or nearly lattice-matched to the substrate (101), transparent to the generated light, and doped by an acceptor impurity.
The p-contact layer (109) is preferably formed from a material lattice-matched or nearly lattice matched to the substrate, is transparent to the generated light, and is doped by an acceptor impurity. The doping level is preferably higher than that in the p-cladding layer (129).
The metal contacts (111) (shown in FIG. 1(c)) and (112) are preferably formed from the multi-layered metal structures. The metal n-contact (111) is preferably formed from a structure including, but not limited to the structure Ni—Au—Ge. Metal p-contacts (112) are preferably formed from a structure including, but not limited to, the structure Ti—Pt—Au.
A window is formed on the back side of the substrate, where no bottom, or n-contact (111) is deposited, and the back substrate surface is mirror-like.
The confinement layer (125) is formed from a material lattice-matched or nearly lattice-matched to the substrate (101), is transparent to the generated light, and is either undoped or weakly doped. The active region is preferably placed within the confinement layer (125) is preferably formed by any insertion, the energy band gap of which is narrower than that of the layers (122), (124), (126) and (129). Possible active regions include, but are not limited to, a double heterostructure, a single-layer or a multi-layer system of quantum wells, quantum wires, quantum dots, or any combination thereof. In the case of a device on a GaAs-substrate, examples of the active region include, but are not limited to, a system of insertions of InAs, In1-xGaxAs, InxGa1-x-yAlyAs, InxGa1-xAs1-yNy or similar materials.
Highly reflecting coat (117) is preferably mounted on the rear facet of the device, and an anti-reflecting coat (116) is preferably mounted on the front facet of the device.
The device operates as follows. The active region generates gain, when a forward bias is applied. Light (104) generated in the leaky waveguide (120) leaks to the substrate (101). Light in the substrate propagates (134) at a certain leaky angle leaky to the plane of the substrate surface. Light is reflected back from the back surface (131) of the substrate. Thus, in addition to the central part of the waveguide (120), wherein (120) may be considered as a first cavity, a second cavity is formed between the leaky waveguide (120) and the back surface of the substrate (131). Since the thickness of the substrate significantly exceeds the wavelength of light in the vacuum (preferred wavelengths of light range between 300 nm and 3 μm), the propagation of light in the substrate obeys the laws of geometrical optics. Therefore, in order to allow the exit of light from the substrate through the facet, it is necessary that the leaky angle leaky is below the angle of the total internal reflection at the semiconductor-air interface. Then, light comes out (145) through the front facet forming preferably a two-lobe far-field pattern with narrow lobes.
If the back surface of the substrate is polished, the light reflects back to the active region layer and no significant part of the light is lost. The threshold current density is low, even if the nominal leakage loss from the waveguide (120) is high. Moreover, the light coming back from the substrate to the waveguide (103), interferes with light (104) propagating just along the waveguide (103). When phase matching conditions hold, constructive interference between light propagating in the waveguide (103) and light returned from the substrate occurs. The phase matching conditions are met only at certain wavelengths which results in wavelength selectivity. In different approaches, the back side of the substrate may be coated, etching may be applied to enable wavelength adjustment, gratings can be deposited to additionally improve wavelength stabilization or enabling grating outcoupling of the light through the substrate, and so on. One or a few coatings can be deposited on the back surface of the substrate to protect the mirror-like quality of the surface.
FIG. 1(c) shows a schematic diagram of a device with a reflection from the substrate surface with an example of one of possible processing layouts, where selective deposition of the bottom n-type contact (111) leaves some parts of the back substrate surface (131) uncovered forming a mirror like semiconductor/air interface enabling a mirror like reflection of light from the back side of the substrate.
The lasing of the device (100) occurs, when the phase matching conditions between the light (104) propagating along the waveguide (103) and light leaking to the substrate, propagating in the substrate (134), reflecting back from the back surface of the substrate (131), and returning to the active region (125), are met. This occurs only for selected vertical optical modes and for selected wavelengths, which enables both single vertical mode operation and wavelength-selective operation.
An alternative way of creating a single-mode vertical lasing from a broad vertical waveguide has been taught in the U.S. Pat. No. 7,583,712, filed Jan. 3, 2007, entitled “OPTOELECTRONIC DEVICE AND METHOD OF MAKING SAME”, issued Sep. 1, 2009, by the inventors of the present invention, this patent being incorporated herein by reference. Instead of leakage of light to the substrate, leakage of light into a second cavity is employed. FIG. 2 illustrates schematically a laser (200) comprising a first cavity, or a first waveguide (253) and a second cavity, or a second waveguide (203) separated by an intermediate cladding layer (250). The first waveguide (253) is thus bounded by the top cladding layer (258) and the intermediate cladding layer (250). The second waveguide is bounded by the bottom cladding layer (202) and the intermediate cladding layer (250). The active region (256) is placed within the first waveguide (253). The device (200) is preferably grown on an n-doped substrate (201). The bottom cladding layer (202), the second cavity (203), the intermediate cladding layer (250), and the layer (254) are preferably n-doped. The layer (257) and the top cladding layer (258) are preferably p-doped. The heavily p-doped p-contact layer (109) is placed on top of the top cladding layer (258). The bottom n-type contact (111) is deposited on the back side of the substrate. The top p-type contact (112) is deposited on top of the p-contact layer (109).
The active region (256) generates light when a forward bias (213) is applied. Light generated in the active region leaks from the first waveguide (253) through the intermediate cladding layer (250) into the second waveguide (203), propagates in the second waveguide (203), is reflected back from the bottom cladding layer (202) and returns back to the first waveguide (253). Lasing occurs when the phase matching conditions are met between light propagating just in the first waveguide (253) and light leaking to the second waveguide (203) and returning back.
FIG. 2 illustrate a tilted optical mode (270) in the first waveguide (253) and a tilted optical mode (220) in the second waveguide (203). The interaction between the optical modes of two cavities results in the formation of a combined optical mode of the entire device (200). The thickness of the first waveguide (253) does not preferably exceed three times the wavelength of light in the vacuum, whereas the second waveguide (203) is much broader, and can reach typical dimensions of 10 to 30 micrometers. Such extension of the second waveguide leads to the laser emission in two narrow vertical lobes (245).
FIG. 3(a) illustrates the principle of the wavelength selection. Referring to the laser (200), each of the two waveguides (253) and (203), confines optical modes. Optical modes confined in each of the waveguides may be described by dispersion laws relating the wavelength of light in the optical mode and the effective tilt angle of the mode. The wavelength of the optical mode confined in the narrower waveguide (253), as a function of the mode angle , is described by a solid curve in FIG. 3(a). The wavelength of the optical modes confined in the broader waveguide (203), as a function of the mode angle , is given by dashed curves. The phase matching condition for the device (200) is met at an intersection point of the two curves. As the thickness of the broader waveguide (203) significantly exceeds the thickness of the narrower waveguide (253), the spacing between the optical modes of the broader waveguide (203) in FIG. 3(a) is smaller than the spacing between the modes of the narrower waveguide (253). The laser (200) generates laser light at one or a few selected wavelengths, at which phase matching conditions are met, and a constructive interference and, hence, a positive feedback occurs. Namely, these are wavelengths 1, 2 and 3 in FIG. 3(a). The spectrum of laser radiation is also illustrated in FIG. 3(b). If only one selected wavelength overlaps with the luminescence spectrum of the active region (256) of the device (200), the device will generate wavelength-stabilized laser light.
The interaction between the optical modes confined in the first, narrow waveguide (253) and the modes of the second, broad waveguide (203) determines the optical mode of the entire device (200) and, hence, the far field pattern of the laser radiation. The far field pattern is shown schematically in FIG. 3(c). Generally, the far field pattern contains two narrow lobes generated from the broad waveguide (203) and a broad angular spectrum generated from the narrow waveguide (253). If narrow far field is targeted, the narrow waveguide (253) and the leaky angle  are engineered preferably in such a way, to ensure that most of the optical power is emitted in the narrow lobes. Preferably, more than eighty percent of the total optical power is emitted in the narrow lobes.
The same applies to a laser (100) of FIG. 1, wherein the substrate is employed as a broad waveguide.
If only a single lobe emission is targeted, this can be achieved by employing external mirrors. FIG. 4 shows a schematic diagram of an apparatus (400) enabling laser emission in a single narrow vertical lobe. Primarily, the laser (100) emits light which has a two-lobe far field (145). Correspondingly, two collecting mirrors (441) and (442) are used to reflect light back (146) to the facet. One of the mirrors (441) may be chosen semi-transparent to allow laser light coming out (445) having a single lobe far field. The apparatus (400) comprises an edge-emitting device (100), a first cavity between the facet (407) and the mirror (441), a second cavity between the facet (407) and the mirror (442), a non-transparent collecting mirror (442), and a semi-transparent collecting mirror (441).
Similarly, laser light in a single narrow lobe may be obtained from an apparatus, combining a laser (200) of FIG. 2 and a similar set of external mirrors.
Another possibility of a laser having an extended waveguide is a laser based on a vertical photonic band crystal disclosed in the U.S. Pat. No. 6,804,280, filed Sep. 4, 2001, entitled “SEMICONDUCTOR LASER BASED ON THE EFFECT OF PHOTONIC BAND GAP CRYSTAL-MEDIATED FILTRATION OF HIGHER MODES OF LASER RADIATION AND METHOD OF MAKING THE SAME”, issued Oct. 12, 2004, by the inventors of the present invention, whereas this patent is incorporated herein by reference.
Yet another possibility of a laser having an extended waveguide is a tilted cavity laser disclosed in the U.S. Pat. No. 7,031,360, filed Feb. 12, 2002, entitled “TILTED CAVITY SEMICONDUCTOR LASER (TCSL) AND METHOD OF MAKING SAME”, issued Apr. 18, 2006, by the inventors of the present invention, whereas this patent is incorporated herein by reference.
In what follows, a tilted wave laser will be referred to as an example. A one skilled in the art will appreciate that the present invention refers to all types of semiconductor lasers having an extended waveguide.
FIGS. 5(a) and 5(b) show schematic diagrams comparing two basic realizations of the tilted wave laser. Both comprise a narrow waveguide and a broad waveguide coupled via a thin cladding layer. In FIG. 5(a) the substrate is employed as a broad waveguide. In FIG. 5(b) the broad waveguide is a layer grown epitaxially on a substrate. The functionality of both types of the tilted wave laser is basically the same. The only important difference is the thickness of the broad waveguide. A typical thickness of a waveguide grown epitaxially on a substrate may be within a range from 5 micrometers to 30 micrometers. The thickness of a substrate is typically within a range from 50 micrometers to 300 micrometers.
The advantage of using a substrate as a broad waveguide is a much simpler epitaxial growth and a larger thickness, if needed. The disadvantage of using a substrate as a broad waveguide is a necessity of special processing, e.g. as shown in FIG. 1(c) in order to reach mirror-like reflection of light from the back surface.
The advantage of using an epitaxial layer as a broad waveguide is a high quality epitaxial interface between the broad waveguide and bottom thick cladding layer which ensures the mirror-like reflection of light. Visible disadvantages are a need to grow epitaxially a thick layer which adds to the costs and limitations on the thickness.
If the targeted wavelength of light is such that the typical commercially available substrates are not transparent (e.g., GaAs substrate is absorbing at the wavelengths λ<870 nm), then only such a device is possible where a broad waveguide is realized as an epitaxial layer of a different material (e.g., as a transparent GaAlAs alloy).
FIG. 6 shows the refractive index profile and the optical mode profile for an example tilted wave laser, wherein the 100 micrometer-thick substrate is employed as a broad waveguide. FIG. 6(a) shows the refractive index profile on a coarse scale. FIG. 6(b) demonstrates the profile of the optical mode, showing specifically the electric field strength profile (the near-field profile). The near field profile reveals a strong peak within the narrow waveguide and an oscillatory behavior in the substrate. FIG. 6(c) shows the refractive index profile and the optical mode profile on a fine scale within the narrow waveguide.
FIGS. 7(a) and 7(b) show the far-field pattern of the optical mode of FIG. 6(b). FIG. 7(a) shows the far-field pattern on a linear scale, and FIG. 7(b) shows the far-field pattern on a logarithmic scale allowing identify all features. The far-field pattern contains two narrow lobes, an oscillatory background, and a broad angular spectrum (central lobe). The two narrow lobes originate from the tilted wave in the broad waveguide. The oscillatory background comes from a finite thickness of the broad waveguide. The broad angular spectrum originates from the light in the narrow waveguide.
FIG. 8(a) shows schematically another possibility of lasing from a laser comprising a narrow waveguide (820) coupled to a substrate (101) through a thin cladding layer (822), a so called leaky laser (800). Light generated in the active medium located in the narrow waveguide leaks to the substrate (834) and then is emitted (845) from the substrate through the facet. Furthermore, light is also emitted (841) from the narrow waveguide (820). In this case no significant reflection from the back side of the substrate and no return of light to the narrow waveguide occurs. Light is emitted in the form of a single narrow lobe (855) and a broad angular spectrum (851) as shown in FIG. 8(b).
In a real device both types of lasing, one in the tilted mode as shown in FIG. 3(a) and one in the leaky mode as shown in FIG. 8(b) can coexist. In certain regimes, the lasing can start at threshold as tilted wave lasing, but, as the drive current increase, the temperature in the active region increases thus increasing the refractive index. This leads to localization of the optical mode and to enhancement of the emission in a broad angular peak.
Once the high power high brightness lasing is required for certain applications, the goal is to improve the characteristics of a tilted wave lasers such that the relative fraction of the output power in the narrow beams is maximized and the relative fraction of the output power in the broad angular spectrum emitted from a narrow waveguide is minimized. Thus, there is a need in the art to further improve the beam quality of the tilted wave laser and stabilize the tilted wave lasing in the two narrow vertical beams The present invention discloses means to additionally configure the tilted wave laser specifically to attain this goal. Furthermore, wavelength stabilized operation can occur only in a tilted wave mode. Therefore, is the wavelength-stabilized operation of a laser is a goal, the lasing in a mode of the narrow waveguide must be suppressed.